Internet Core Protocols: The Definitive Guide

Exchanging data

Once a virtual circuit has been established, the applications in use can begin exchanging data with each other. However, it is important to note that applications do not exchange data directly. Rather, each application hands data to its local TCP provider, identifying the specific destination socket that the data is for, and TCP does the rest.

Applications can pass data to TCP in chunks or as a contiguous byte-stream. Most TCP implementations provide a “write” service that is restricted in size, forcing applications to write data in blocks, just as if they were writing data to a file on the local hard drive. However, TCP’s buffering design also supports application writes that are contiguous, and this design is used in a handful of implementations.

TCP stores the data that it receives into a local send buffer. Periodically, a chunk of data will get sent to the destination system. The recipient TCP software will then store this data into a receive buffer, where it will be eventually passed to the destination application.

For example, whenever a web browser issues an HTTP “GET” request, the request is passed to TCP as application data. TCP stores the data into a send buffer, packaging it up with any other data that is bound for the destination socket. The data then gets bundled into an IP datagram and sent to the destination system. The recipient’s TCP provider then takes the data and passes it up to the web server, which fetches the requested document and hands it ...